To mitigate greenhouse gas emissions, coal-fired electricity infrastructure needs to be replaced by low-carbon electricity generation options. Here we examine a range of possible alternative scenarios for sustainable electricity generation in South Korea, considering both physical and economic limits of current technologies. The results show that South Korea cannot achieve a 100% renewable energy mix and requires at least 55 GW of backup capacity. Given that constraint, we modelled seven scenarios: (i) the present condition, (ii) the First National Electricity Plan configuration, (iii) renewable energy (including 5 GW photovoltaic) with fuel cells or (iv) natural gas backup, (v) maximum renewable energy (including 75 GW photovoltaic) with natural gas, (vi) maximum nuclear power, and (vii) nuclear power with natural gas. We then quantify levelised cost of electricity, energy security, greenhouse gas emissions, fresh water consumption, heated water discharge, land transformation, air pollutant emissions, radioactive waste disposal, solid waste disposal and safety issues for each modelled mix. Our analysis shows that the maximum nuclear power scenario yields the fewest overall negative impacts, and the maximum renewable energy scenario with fuel cells would have the highest negative impacts.